A negative glow discharge lamp typically is comprised of a light transmitting envelope containing a noble gas and mercury with a phosphor coating on an inner surface of the envelope which is adapted to emit visible light upon absorption of ultraviolet radiation that occurs when the lamp is excited. The lamp is excited by means of the application of a voltage between the lamp electrodes. At least one of the electrodes is in the form of an electron emissive cathode. In a d.c. operated preheat-type lamp having an anode and cathode, the cathode is preheated to electron emitting temperature for several seconds. Current flows between the electrodes after a certain potential is applied to the electrodes, commonly referred to as the breakdown voltage. An elementary explanation of the phenomenon is that the gas between the electrodes becomes ionized at a certain voltage, conducts current and emit ultraviolet radiation. The ultraviolet radiation is converted to visible radiation by means of a phosphor layer disposed on the inner surface of the lamp envelope. It is understood that what is meant by a negative glow discharge lamp, as distinguished from a positive column lamp, is one in which the anode is positioned so that no appreciable positive column is developed within the discharge.
During operation of a fluorescent lamp, it is advantageous to produce a double hot spot on the cathode so as to lower the electrode temperature. The reduced electrode temperature has been found to cause an improvement in the lumen maintenance and life of the lamp. U.S. Pat. No. 2,337,993, which issued to Hall, Jr., et al on Dec. 28, 1943, discloses a lamp comprising a plurality of cathodes arranged and combined in a manner to provide a plurality of emissive or hot spots on each cathode of each electrode. This patent requires special circuitry to operate a lamp wherein each end electrode has a plurality of cathodes.
An article entitled "High Frequency Operation Producing Double Hot Spots on Electrodes for Fluorescent Lamps" in Journal of the Illuminating Engineering Society (Summer 1987) by Yuhara et al lists various methods for producing a double hot spot. The article details, in particular, circuits wherein the frequencies of the lamp current and the filament voltage are different.
Thermal switches have been used in rapid-start and preheat-type fluorescent lamp for various purposes. For example, U.S. Pat. No. 4,616,156, which issued to Roche et al on Oct. 7, 1986 discloses a rapid-start lamp containing a thermal switch. The thermal switch is connected in series with the cathode for discontinuing heater current upon operation of the fluorescent lamp to reduce energy requirements. U.S. Pat. No. 2,351,305, which issued to Thayer on June 13, 1944, discloses a preheat-type fluorescent lamp having a thermal switch located within the lamp base structure. The thermal switch is connected in series with the cathode filament to provide preheating current to the filament prior to lamp ignition.